Electrophotographic development is a method wherein an image is developed by causing toner particles present in a developer to adhere to an electrostatic latent image on a photosensitive material. The developers utilized in this method are classified into two-component developers composed of toner particles and carrier particles, and one-component developers consisting of toner particles.
The cascade development process is a traditional method in which the two-component developers composed of toner particles and carrier particles are used, but the magnetic brush development process involving a magnet roller is currently mainstream.
The carrier particles in the two-component developers give a desired electric charge to the toner particles when they are stirred together in a developing box filled with the developer, and further work as a carrier substance that transports the charged toner particles onto the surface of a photosensitive material to form a toner image on the photosensitive material. The carrier particles which remain on a magnet-having developing roller are reintroduced into the developing box, and the particles are mixed and stirred with new toner particles. In this way, the carrier particles are used repeatedly for a certain period.
The two-component developers differ from the one-component types in that the carrier particles are mixed and stirred with the toner particles to charge and transport the toner particles, permitting good controllability in designing the developer. Therefore, the two-component developers are suited for use in full-color printers requiring high image quality and high-speed printers requiring image-maintaining reliability and durability.
It is necessary that the two-component developers show desired levels of image properties such as image density, blushing, white spots, tone properties and resolving power from an initial stage and that the properties are stably maintained without change throughout long term use. In order that these properties are maintained stably, the carrier particles contained in the two-component developers must have stable properties.
Iron powder carriers such as oxide-coated or resin-coated iron powders are conventional carrier particles in the two-component developers. The iron powder carriers possess high magnetization and high conductivity to provide advantageous easy and highly reproducible development of solid images.
The iron powder carriers, however, have a heavy own weight and its magnetization is too high, so that when they are stirred and mixed with toner particles in a developing box, the toner is more likely to adhere to the surface of the iron powder carrier, that is, toner-spent is apt to occur. The toner-spent will lead to decrease of the effective carrier surface area and lowering of the capability of triboelectrically charging the toner particles.
With regard to the resin-coated iron powder carriers, the surface resin is often separated by stress during long-time use and consequently the core (iron powder) having high conductivity and low dielectric breakdown voltage is exposed to induce leakage of electric charge. The leakage of electric charge breaks an electrostatic latent image formed on a photosensitive material to cause brush traces and the like in solid images, and therefore development of uniform images is difficult. Such inferior durability has been a problem of the resin-coated iron powder particles. From the above reasons, the iron powder carriers including the oxide-coated iron powder and the resin-coated iron powder have been avoided.
As a substitute for the iron powder carriers, there have recently been proposed magnetic powder-dispersed resin-binder carriers in which magnetic powder is dispersed in a binder resin. The magnetic powder-dispersed resin-binder carriers have been insufficient in durability in long-term use. JP-A-2000-199985 proposes a resin-coated magnetic carrier that includes magnetic powder whose surface is lipophilized to give a functional group, wherein the functional group on the magnetic powder surface is chemically bonded with a functional group of the coating resin. This resin-coated magnetic carrier has a problem that the carrier core is inferior in mechanical strength because the binder resin is not a silicone resin synthesized by ring-opening addition reaction free of by-products. Further, the chemical bonds between the functional groups of the magnetic powder surface and coating resin as illustrated in FIG. 3 cannot achieve a sufficient bond strength between the core and the coating resin layer because there is only a small number of functional groups exposed on the core surface and consequently there are few chemical bonds with the functional groups of the coating resin. Accordingly, the coating durability has been unable to meet the recent difficult requirements.
Meanwhile, magnetic powder-dispersed carrier cores that are composed of binder resins such as phenolic resin possessing higher surface free energy than silicone resin have another problem that if the carrier core has an exposed part, it is easily contaminated with the toner or the like during printing to cause defective images.
Further, when the carrier core is formed of a binder resin such as silicone resin having low surface energy and the coating resin has no functional groups enabling chemical bond with the core-forming binder resin, the resin coating step tends to result in poor adhesion of the coating layer because of few chemical bonds. Thus, it has been difficult to obtain resin-coated carriers based on magnetic powder-dispersed binder resin with sufficient coating durability.
Accordingly, there has been a need for a resin-coated carrier based on magnetic powder-dispersed binder resin that has high bond strength between the core and the coating resin layer and is free of separation of the coating resin layer from the core.
JP-A-05-113696 discloses magnetic particles in which a polymer forming a core and a polymer forming a coating layer are covalently bonded. These polymers are each formed from a radically polymerizable monomer, and therefore the production steps are intricate and the polymerization must be performed in an inactive atmosphere, causing industrial disadvantages. Furthermore, when the magnetic powder is exposed on the surface of the resin-coated carrier, the magnetic powder is apt to be released and the carrier particles tend to be contaminated with the toner, leading to a damaged drum and defective images such as blushing.
Moreover, the coating resins are limited to resins formed from radically polymerizable monomers, which narrows the degree of designing freedom to satisfy carrier properties required and makes it difficult to cope with varied requirements. The magnetic particles of JP-A-05-113696 differ from the carrier for electrophotographic developer of the present invention in that both the core and the coating layer of JP-A-05-113696 contain magnetic powder.
It is an object of the present invention to provide a carrier for electrophotographic developer that comprises a carrier core comprising a binder resin and magnetic powder dispersed in the resin, and a coating resin layer formed on the core surface, wherein the carrier is free of release of the magnetic powder, has high mechanical strength and good environmental stability, can prevent the toner-spent, possesses excellent flowability and high bond strength between the core and coating resin layer, and thereby achieves superior durability and toner-charging capability.
It is another object of the invention to provide a two-component electrophotographic developer that contains the carrier particles having the above properties.